Try typing "about:" in the URL bar by press-and-hold on 'b' (not through the symbols screen). You'll get "about:b". Key-hold inserts colon. Key-up inserts 'b'. Doesn't happen to numeric or other symbol characters. Tested on Aurora 19 from 2012-11-26.
I got an idea on how to fix it after talking to Brad.
Turns out my idea didn't work. Basically the bug occurs because, as soon as you enter a colon in the awesomebar, we tell the keyboard to switch to URL mode (e.g. some keyboards add ".com" key in URL mode). This happens even if the user is still pressing down the key like in SwiftKey's case. SwiftKey has this behavior that when it switches modes, it forgets about the fact that you were pressing down to enter colon, not 'b'. So when you release the key, SwiftKey happily enters the letter 'b' as well. My idea was to switch to URL mode after you've released the key, but that requires SwiftKey to tell us when the key has been released. Turns out SwiftKey doesn't do that, and I could not find a good way to detect when the key is released. I think it's more sensible for SwiftKey to fix the bug (i.e. when it switches modes and a key is down, it shouldn't act when the key comes up). Otherwise, I don't really have an idea on how to properly fix this.
In the mean time, the workaround fix for bug 827325 also fixes this bug for me, so I'm marking this as resolved. Fix is in the latest Nightly and Aurora.